In recent years, 3GPP (3rd Generation Partnership Projects) has specified a mobile communication system called a W-CDMA (Wideband-Code Division Multiple Access) system (or a UMTS: Universal Mobile Telecommunications System) based on CDMA (Code Division Multiple Access) technologies.
There has also been specified a system for increasing a data transmission rate in the W-CDMA system. A system for increasing a downlink data transmission rate is called “HSDPA (High Speed Downlink Packet Access)”.
In the W-CDMA system, a mobile station UE (User Equipment) has communication states, which are called a “CELL_FACH” state and a “CELL_DCH” state.
In the “CELL_FACH” state, multiple mobile stations UEs execute communication while sharing one radio channel (hereinafter referred to as an FACH: Forward Access Channel). The “CELL_FACH” state is often used when a high transmission rate is not required. However, the “CELL_FACH” state achieves high radio resource utilization efficiency since the multiple mobile stations UE use one FACH as needed.
The “CELL_DCH” state is different from the “CELL_FACH” state, and is a state where one mobile station UE occupies one radio channel (hereinafter referred to as a DCH: Dedicated Channel) for communication. In the “CELL_DCH” state, each of the mobile stations UE can stably perform relatively high-speed communication, and also serviceability is high. However, the radio resource utilization efficiency is not good since each mobile station UE occupies the corresponding DCH even if the mobile station UE has no data to be transmitted.
In the “CELL_DCH” state, uplink DCCH (Dedicated Control Channel) data (uplink control signal data), downlink DCCH data (downlink control signal data), uplink DTCH (Dedicated Traffic Channel) data (uplink user data or uplink U-Plane data) or downlink DTCH data (downlink user data or downlink U-Plane data) is transmitted on the DCH.
Meanwhile, in the HSDPA, a shared radio channel (hereinafter referred to as a HS-DSCH: High Speed-Downlink Shared Channel) is used to transmit downlink DTCH data, as in the case of the “CELL_FACH” state.
The HSDPA enables the efficient use of radio resources while increasing the downlink data transmission rate by following changes in propagation environment so quickly with high-speed control in the lower layer and by controlling the downlink data transmission rate according to the radio quality of each mobile station UE.
Moreover, in the HSDPA, the uplink DCCH data, the downlink DCCH data or the uplink DTCH data is transmitted on the DCH (A-DPCH to be described later), while the downlink DTCH data is transmitted on the HS-DSCH.
Here, the DCH for transmitting the uplink DCCH data, the downlink DCCH data or the uplink DTCH data is called “A-DPCH (Associated Dedicated Physical Channel)”, meaning “DCH associated with the HS-DSCH”.
In the HSDPA, there has been specified a method for transmitting downlink DCCH data using not the A-DPCH but the HS-DSCH. Such a method can contribute to the improvement in radio resource utilization efficiency, and can also reduce connection delay time for establishing a radio channel between the mobile station UE and a radio controller by transmitting the downlink DCCH data at high speed.
Note that when transmitting the downlink DCCH data using the HS-DSCH, only data notifying transmission power in uplink communication (pilot signal data for transmission power control, hereinafter referred to as TPC (Transmission Power Control) bits) needs to be transmitted through another downlink channel in order to follow changes in propagation environment at high speed.
Such a downlink channel for transmitting only the TPC bits is called an “F-DPCH (Fractional-DPCH)”, and ten mobile stations UE can be multiplexed on one F-DPCH. When the F-DPCH is used, CC (Channelisation Code) utilization efficiency is improved by up to ten times comparing with the case where the A-DPCH is used.
Moreover, in the W-CDMA system, an “RLC (Radio Link Control) function” is specified as a layer-2 function for radio link control. Here, the RLC function is configured to execute delivery confirmation and retransmission control.
A data size (hereinafter referred to as RLC-PDU size) of each of the blocks separated by the RLC function is usually 42 octets, but may be 82 octets. Setting the RLC-PDU size to 82 octets enables more data to be transmitted per unit time, thereby enabling an increase in data transmission rate.
As a result of the consideration that the RLC-PDU size is set to 82 octets to further improve the downlink data transmission rate, communication using the RLC-PDU size of 82 octets can be performed using either the transmission method using the A-DPCH or the transmission method using the F-DPCH. Note that, in the “CELL_FACH” state or the “CELL_DCH” state, a specified transmission rate can be sufficiently satisfied even if the RLC-PDU size is 42 octets.
In such a mobile communication system, the radio controller generally performs channel switch control to allocate a suitable traffic channel to the mobile station UE according to the usage situation of the radio resources.
For example, the radio controller observes a flow rate of user data to the mobile station UE, and performs the channel switch control in the following manner. Specifically, if the flow rate of the user data to the mobile station US is high, the mobile station UE performs communication using the OCR or the HS-DSCH. On the other hand, if the flow rate of the user data to the mobile station UE is low, the mobile station UE performs communication using the FACH.